Use of composite materials in the repair of both accessible and inaccessible piping systems is becoming increasingly popular. The costs associated with replacing new conduits may be avoided or at least delayed by carrying out maintenance procedures at the damaged section of an on site or in-situ conduit. Generally, such maintenance procedures entail locating the damaged section and installing a thin, durable material to cover the defects, thus restoring the integrity of the conduit.
The materials and procedures employed in in-situ repair technology have been quite varied. Both low-cost, low-quality and high-cost, high-quality composites have been developed. Most composites are designed as highly flexible materials in order to facilitate their transport to the damaged vicinity of the conduit. After transport, means are employed to conform the composite to the internal geometry of the damaged section. A reinforced lining impregnated with a resin covering the damaged section is then permanently cured to form a protective shell. The lining may be impregnated with resin on site or pre-impregnated at a remote location. The curing is accomplished either by ambient conditions or by positive heat-activation methods, such as hot water, steam, or electrical resistance heating. Ambient curing is inferior, however, because ambient conditions may vary widely and disrupt the curing cycle.
In the past, flexible heaters have been produced using ferrous or metallic wires within the composition to provide heat by resistive means. While these wires are an efficient heating element, the flexibility of the heater is limited by the use of such wires. For instance, in Japan 2158323 copper wires are used as the heating elements. With the repeated inflating and deflating that would be experienced with repeated use, it is expected that the redundant load paths associated with the flexing will cause the copper wires to fail, thus losing electrical continuity and heating capability. This severally limits the life cycle of a flexible heater manufactured with metallic wires. Copper wires disposed in a flexible composition also exhibit very poor adhesion to the surrounding polymer (usually silicon) making uniform and consistent positioning of the wires within the polymer matrix, throughout the expected life cycle of the heater, difficult if not impossible. This can result in the resistance wires being redistributed within the heater in undesired arrangements. While various primers can be employed to increase the bond strength between the polymer matrix and the wires, such primers can further degrade the flexible strength of the wire and limit its malleability, causing premature failure. Additionally, as copper or metallic wires are heated (resistively), their electrical resistance increases proportionately to the temperature increase. In a flexible heater, this means that the amount of power required to achieve a desired temperature must be increased throughout the heating cycle. The relatively high mass of copper or other electrically conductive metal, also results in a lag in response time when used as a heating element, thus requiring constant monitoring and adjustment of the power supply.
Inflatable bladders that incorporate various heating means have also been used for curing materials impregnated with a thermosetting resin matrix, such as polyester or epoxy based resins. In these resin types, certain chemicals are present that have a detrimental effect on silicone products. Specifically, silicones, when exposed to certain chemicals such as styrene, which is present in many resin systems, and heat, will revert after a limited number of uses to into a weak form no longer suitable as an inflation device.
Historically, the actual production of flexible, inflatable heaters has been accomplished by various means. In one method, uncured strips of resin impregnated sheets of resilient, flexible material are laid on a mandrel or forming surface. The strips are then exposed to a heat source capable of providing sufficient heat to cure the uncured strips. Ovens have been used to accomplish this curing procedure. However, the ovens required are expensive and generally inefficient, as they must heat a large volume of air as well as the mandrel or forming surface, and the uncured strips. Depending on the mass of the mandrel, a substantial cool down period must also be observed before the heater can be removed from the mandrel. Considerable energy is lost to the atmosphere and cycle times are lengthy in such a procedure. This translates into excessively high manufacturing costs.
In view of the aforementioned shortcomings associated with the conventional methods of construction and use of flexible, inflatable heaters, there is a strong need for a inflatable heating device containing a heating mechanism that is robust. There is also a strong need for materials that can withstand repeated use in aggressive environments and afford a long life cycle. It will be appreciated that there is also a strong need for an improvement in manufacturing which can reduce production cycle time and capital equipment costs.
The present invention has been developed in response to a need for improved yet affordable composite materials for use as an inflatable flexible heater, as well as for a need for improved in-situ repair procedures.